Trouble using a volume

So, I am the beginner. I have an external HD that I want to create a 40gb volume. I walked through the steps. I created a file name within the hard drive, and all steps seemed to work just fine. I then mounted the volume and could then see it in my list
of drives. However, when I attempt to open the drive I get the notice that I need to format the drive. That was done during the creation process.